I disagree. In my general experience, "Mama" is usually accepted as the name of one's Mom, but not as a descriptive noun. "My Mom" is far more usual than "my mama", and I'd go so far as to suggest that "my mama" is a vernacular peculiar to certain regions of the country, mostly rural. It is not common colloquial English, and in some other regions, saying "my mama" would mark you as unsophisticated, perhaps under-educated.
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